We are genetically programmed to check out potential mates to procreate with, men more overtly.
A man will look for large breaststroke and hips denoting potential birthing qualities, a woman will look for muscles and strength for protection, and good sized genitals for a good provider of babies.
Civilisation has diluted most of the instincts, bit they are still there, in absolutely general terms, and with no inference that all men and all women do this 'checking' simply that it is an anthropological hangover from our caveman days.